The purpose of this study is to determine the comparability of Azithromycin plus m-Atovaquone to TMP/SMX in the prevention of serious bacterial infections; to determine and compare the long-term safety and tolerance of TMP/SMX vs. m-Atovaquone/Azithromycin; and to look at the likelihood of a pharmacokinetic interaction between m-Atovaquone and Azithromycin. Targeted accrual for this study is a total of 690 patients nationwide, with 340 patients on each treatment regime.